Electric AC motors such as are employed in electric vehicles, for example, usually require a converter that converts the DC voltage provided by a battery into an AC voltage. Conventional converters in such vehicles use so-called bridge circuits that alternately connect output terminals to a positive and a negative pole of the DC voltage source. In each switching state, the converters choose the residence duration such that the demanded AC voltage arises on average over time. However, an AC voltage generated in this way has a low quality and distortion. In addition, high energy loses occur as a result of the switching processes. Further disadvantages occur in terms of the electromagnetic compatibility, since the high-frequency switching edges that arise as a result of the switching cause high energies to be emitted electromagnetically. Moreover, the circuits necessitate expensive components since the latter have to be designed in each case for the peak voltage.
The problems mentioned can be combated using modular multilevel converters. Modular multilevel converters are known inter alia from “A. Lesnicar, R. Marquardt (2003), An innovative modular multilevel converter topology suitable for a wide power range, IEEE Power Tech Conference Proc., 3:6ff.”, which is incorporated by reference herein, “M. Glinka, R. Marquardt (2005), A new AC/AC multilevel converter family, IEEE Transactions on Industrial Electronics, 52:662-669, which is incorporated by reference herein and “S. M. Goetz, A. V. Peterchev, Th. Weyh (2015), Modular multilevel converter with series and parallel module connectivity: topology and control, IEEE Transactions on Power Electronics, 30(1):203-215,” which is incorporated by reference herein. Electrical converter systems are likewise known from DE 10 2011 108 920 A1, which is incorporated by reference herein and DE 10 2010 052 934 A1, which is incorporated by reference herein.
Modular multilevel converters allow the output voltage for a load, such as an electric AC motor, to be generated in small stages. In modular multilevel converters, individual modules each comprising an energy storage element and a plurality of switching elements are electrically interconnected with neighboring modules, wherein the electrical interconnection is dynamically freely variable during operation, such that the output voltage is generated by dynamically changing serial and parallel connection of the energy storage elements. The individual modules constitute extra-low-voltage sources which are hardwired among one another and which can be varied in terms of their voltage and can be electrically connected to the other extra-low-voltage sources. A multiplicity of arrangements of the switching elements and of the energy storage element have been developed for the individual modules. A respective arrangement of the energy storage element and the switching elements is referred to as a microtopology.
However, modular multilevel converters in the prior art can only dynamically interconnect neighboring modules with one another. By contrast, an arbitrary parallel and serial connection of the electrical energy storage elements of the modules is not possible if the important advantage that the design voltage of the individual components must be only a fraction of the total output voltage is maintained. This gives rise to disadvantages during operation and in terms of the loss behavior of such circuits. The chain structure of the macrotopology, in which modules are strung together in most converter technologies, additionally forces the load current of a converter arm, i.e. of a string of individual modules, to flow through all the modules, as a result of which the ohmic losses of the system are increased unnecessarily.
An omission of modules, particularly in the parallel connection, such that even non-adjacent modules can electrically connect their electrical energy storage elements in parallel among one another, without the omitted module having to be included in the process, is not possible in any technology from the prior art without having to give up in the process the reduced dielectric strength possible for a large portion of the components.
Moreover, other functions, such as e.g. an energy equalization of modules that are not directly adjacent, cannot be realized or can be realized only with high additional circuitry outlay. Furthermore, this requires a high number of semiconductor elements that are usually used because each individual module has to provide all the switching states. The control of large converters comprising a large number of individual modules also poses a problem because all the modules generally have to be driven by a central control.
Hereinafter the term “electrical energy storage element” is intended also to include electrical energy sources and energy sinks which differ from electrical energy storage elements merely in that they preferably enable one area of operation, either an energy uptake or an energy delivery. Furthermore, the electrical energy storage elements designated here need not necessarily be ideal and therefore free of losses, and so the energy that can be drawn may be lower than that previously fed in.
A power converter generally denotes an electrical circuit which can transport electrical energy between a plurality of inputs and in the process affords the possibility of converting current and/or voltage parameters. This includes, in particular, DC-DC converters, inverters and rectifiers.